The present invention relates to a magnetic resonance imaging apparatus (hereafter referred to as an MRI apparatus), and more particularly to a large-scale MRI apparatus having magnetostatic-field-generating magnets for generating strong fields and a magnetic shield for insulating the high fields.
The MRI apparatus to obtain tomographic images of a human body by using the nuclear magnetic resonance phenomenon are widely used in medical institutions. In an examination using the MRI apparatus, magnets are required which generate uniform field strength in a space, where an examined region of an examinee is placed, to produce images reflecting the internal structure of the examined region.
For the magnets of the MRI apparatus, permanent magnets, normal conducting magnets and superconducting magnets have been put into practical use. The superconducting magnets, which can achieve higher magnetostatic field strength, are finding wider applciations than permanent magnets and normal conducting magnets. With MRI apparatus using superconducting magnets capable of providing uniform and high magnetostatic field strength, it has become possible to obtain high-quality images also in examinations by various methods of high-speed photography.
As for types of magnet, long, cylindrical solenoid type magnets have been used. On the other hand, various kinds of the open type magnet devices, which are open at the lateral sides and the front side thereof, have been developed, to get rid of the examinee's feeling or fear of being confined in a narrow space when entering the space between the magnet devices, and also to make it possible to perform interventional operations under the MRI examination as means of monitoring in medical treatment or surgery. Also in the MRI apparatus having such an open type magnet devices, high field magnet devices using super-conducting magnets have been put into practical application to realize high-speed high-quality picture taking. The open type MRI apparatus is disclosed in JP-A-11-197132, JP-A-11-155831 and JP-A-10-179546, for example.
However, as high magnetostatic field strengths have been achieved in the examination space where an examinee is placed, a problem has arisen that the magnetic flux density around the magnet device increases. This problem is conspicuous in the open type MRI apparatus. The magnetic flux density outside the magnet is called leakage field strength and defined by a distance from the magnet center to a location where a flux density of 0.5 mT is measured, and this distance is normally desired to be equal to or less than the size of a room where the MRI apparatus (magnet device) is installed. However, in an open-type MRI apparatus, such as mentioned above, which has high field superconducting magnets of an examination magnetic field density of 0.7 to 1.0 T, this distance is as large as more than 10 m.
To minimize the leakage field to the outside of the examination room, a possible solution is to enclose the walls of the examination room, where the magnets are installed, by a ferromagnetic material. However, to confine the superconducting magnets' leakage field strength of 0.7 to 1.0T within the range of the examination room of an ordinary size (8˜10 m for example), it is necessary to put up a magnetic shield of not less than 10 cm in thickness, which is not practical.
Another solution may be considered to reinforce the magnetic shield by combining the magnets with a magnetic circuit composed of a ferromagnetic material. However, when a ferromagnetic substance is located close to the superconducting coils, there is a possibility that the ferromagnetic substance directly affects the distribution of the flux density in the examination space, and disturbs the field uniformity. If the field uniformity is disturbed, it becomes impossible to perform those imaging methods that require field uniformity of higher mode. For example, in the fat signal suppression method, by using a difference in resonance frequency of about 3˜4 ppm between a water signal and a fat signal, a high luminance signal generated by the fat tissue of the examined region of a human body is suppressed, but this method is not applicable if a higher mode on the z axis occurs as much as 30 ppm.
It is known that attempts to correct a non-uniform field of higher mode by using current shims or miniscule iron pieces are often turned out to be impractical because high current shims or a large number of iron pieces are required for this purpose. Therefore, in the design stage of the magnet devices used in the conventional MRI apparatus, the layout of superconducting coils is decided so as to prevent the occurrence of a non-uniform field of higher mode. However, if one tries to realize superconducting magnets just as designed in the form of open-type superconducting magnets using a large-scale magnetic circuit (iron yoke), there are problems, such as an increased complexity of the structure of the iron yoke, the extreme complexity of processing materials to make the yoke in a monolithic form, and increased waste of materials. Yet another problem is that with the magnet devices using a monolithic iron yoke, it becomes very difficult to mount, inspect and maintain the super-conducting coils.
The weight of the MRI apparatus on completion is often over 10 tons. In such a case, many workers and heavy machinery are required to transport the MRI apparatus to the site and install it. Particularly, in the case of MRI apparatus that generates a high magnetic field by superconducting magnets, because its gross weight is several tens of tons, it should preferably be designed by taking the ease of transport and installation at the site into consideration. At the same time, it is required that the MRI apparatus is so structured as to keep a uniform field throughout the examination space.